Current transformers (CTs), working principle and scope

Functioning energy systems require constant monitoring and the various switching operations. And in order to convert the high-voltage electrical quantities in proportion to the modified analogs and used transformers. In particular to reduce the primary current to acceptable values ​​for the measuring and protective devices used current transformers. On them will be discussed in the above material.

How the current transformer

Work current transformer (CT) is based on the law of electromagnetic induction, which operates in the electric and magnetic fields. They are subject to change in the form of sinusoidal harmonic magnitudes variable nature.

The transformer primary current is transformed into a secondary current vector value in full compliance with the proportionality and retaining angle.

As shown above scheme can realize the processes in running a current transformer. And they look like this:

According to one power winding current flows I1, wherein Z1 overcome the resistance of the winding. Through this process, the coil is formed around the magnetic flux F1, which is trapped magnetic core placed at right angles to the direction of the vector I1. Owing exactly to such arrangement provides the least loss of electrical energy in it in magnetic transformation.

The generated magnetic flux F1 permeates not only power the coil 1, but also penetrates the secondary winding 2. As a result, in the secondary coil induced electromotive force E2, under the influence thereof is formed in two I2, already overcomes coil resistance Z2 and the load impedance Zl. As a result of this process on the secondary winding is formed findings voltage drop U2.

The magnetic flux F2 reduces the magnetic flux F1 and F of the current transformer is formed by adding the two geometric F1 and F2 vectors.

Coefficient current transformer is given by I1 / I2 vectors. And this value is put in the development (production) products. Due to the fact that in the operating CTs current flow is constantly changing, the rate indicated in the nominal form, for example 600/5. This indicates that the CT calculated for a maximum of 600 amperes, and if this current will be in primary organization, the current in the secondary housing to be equal to 5 amperes.

Just the design is laid another very important parameter - the accuracy class, which is characterized by the value of the CT values ​​of the deviations from the actual settlement.

Hazards at work TT

Since magnetic TT implemented electrical steel having excellent conductivity, current it binds two magnetically isolated windings, during operation there is a possibility of damage of the insulating layer, whereby the secondary housing may be at high potential.

Therefore, in order to avoid electrical shocks among staff and preserve the functionality of the connected equipment, one of the terminals of the secondary winding is grounded without fail.

Conclusions secondary winding labeled "I1" and "I2", and the primary winding "A1" and "A2." If the transformer is included in the job, the secondary winding have to be loaded (connected), otherwise it is set shorted.

This should be done because when the current flows through the primary winding having a certain capacity (S = U * I). She is also undergoing a transformation process in secondary values. And if the current circuits severed secondary winding, the value of the current tends to zero, but the voltage is sharply increased and on the contrary open on the secondary housing is formed a high voltage. And this is very dangerous!

Important. That is why all current circuits are to be collected, and unused cores should be set short-circuiting.

TT modification

Industry produces a huge number of variations of the current transformers of various sizes and accuracy classes. But energy widespread combined current transformers, where a single product are combined two core: measuring (0,5 R) and the relay (protective 10R)

By appointment of the current transformers are subdivided into:

1. Intermediate. Intended for re-conversion value.

2. Protective. Connected to the current protection circuits.

3. Laboratory. Having high accuracy class and serve as a screening device.

4. Measuring. Are used to transmit the changed current in proportion to the counters that control the consumed (tempered) electricity.

Also, current transformers are:

- Outer installation intended for installation on OSG (open distribution setting);

- For closed systems installed in indoor switchgear cells (closed distribution installation);

- integrated directly into machinery. For example, in the counters direct connection has its own mini TT;

How to check the CT

The most important possible malfunction operating current transformer is a breakdown of insulation. Therefore, TT workers undergo periodic inspections of such services as:

Insulation Service and Relay Service.

In this case, the insulation service checks TT high voltage, and relay service checks the VAC (volts -Ampernaya Feature), checks the transformation ratio. And if the tests showed that the TT is faulty, then it is rejected and replaced by a new the same rating. Otherwise, the inspection report drawn up:

conclusion

The breadth of use of this product indicates that the current transformers are an essential and, of course, an important part of the whole energy system. If the article was useful to you, or are interested in, you will appreciate it Like. Thank you for your attention!